1. Field of the Invention
The invention relates to pictorial and text scanning apparatus, and more particularly to a method of compensating for non-uniformity in sensors in image scanning.
2. Description of the Prior Art
Historically, copies of document originals have been produced by a xerographic process wherein the document original to be copied is placed on a transparent platen, either by hand or automatically through the use of a document handler, and the document original illuminated by a relatively high intensity light. Image rays reflected from the illuminated document original are focused by a suitable optical system into a previously charged photoconductor, the image light rays functioning to discharge the photoconductor in accordance with the image content of the original to produce a latent electrostatic image of the original on the photoconductor. The latent electrostatic image so produced is thereafter developed by a suitable developer material commonly referred to as toner, and the developed image transferred to a sheet of copy paper brought forward by a suitable feeder. The transferred image is thereafter fixed as by fusing to provide a permanent copy while the photoconductor is cleaned of residual developer preparatory to recharging.
More recently, interest has arisen in electronic imaging where in contrast to the above described xerographic system, the image of the document original is converted to electrical signals or pixels and these signals, which may be processed, transmitted over long distances, and/or stored, are used to produce copies. In such an electronic imaging system, rather than focusing the light image onto a photoreceptor for purposes of discharging a charged surface prior to xerographic development, the optical system focuses the image rays reflected from the document original onto the image reading array which serves to convert the image rays to electrical signals. These signals could be used to create an image by some means such as operating a laser beam to discharge a xerographic photoreceptor, or by operating some direct marking system such as an ink jet or thermal transfer printing system.
The prior art related to these types of systems includes:
U.S. Pat. No. 4,602,293 to Sekine discloses an apparatus having a shading correction reference surface which is scanned prior to scanning a manuscript. The output of a photoelectric converter (i.e., an image sensor array) produced while scanning the shading correction reference surface is converted to a digital signal and stored in a memory as a reference. Subsequently, while the manuscript is scanned, the output of each cell of the photoelectric converter is supplied to a comparator along with an analog version of the corresponding reference signal in the memory to effect shading correction of the output.
U.S. Pat. No. 4,383,275 to Sasaki et al. discloses a system for providing read-out level compensation in an optical reader system. In operation, the system first reads a white background to obtain a reference output. The reference output is reversed and stored in a memory. The actual sensor output derived from an original sheet is multiplied by the memorized, reversed reference output to get a compensated video signal.
U.S. Pat. No. 4,520,395 to Abe discloses a system for correcting shading or non-uniformity in a photosensitive element array due to light source, lens, optical transmission, and sensor characteristics. The system employs a memory having a number of cells corresponding to the number of photoelements positioned along a linear photosensitive array. The sensor output of each respective element is successively compared with data value stored in a corresponding memory cell in the memory. With each successive output of the linear array, the data stored in the memory is updated by determining the larger data value signal and then storing that signal in the corresponding memory cell. The stored data for each line is converted with a weighting factor and multiplied by the sensor output to produce a compensated output.
A difficulty with these prior art systems is often the difference in the sensors or reading elements themselves causing non-uniformity in response, as well as the varying effect on each sensor caused by variations in illumination level caused by optics and lamp degradation. In addition, prior art calibration systems are often limited to manuscript or text applications rather than halftone applications. It would be desirable, therefore, to be able to compensate for the above identified non-uniformities and also be able to provide halftone calibration in addition to text calibration.
It is an object of the present invention, therefore, to provide a new and improved scanner calibration system. It is another object of the present invention to provide an image scanner that calibrates itself by scanning a patch whose density defines the threshold level between black and white. It is another object of the present invention to provide a circuit architecture which gives both thresholding and halftone image processing without a central processing unit. Other advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.